This invention relates to a belt tension adjusting device for adjusting the tension in a power transmission belt such as a timing belt for an automotive engine.
In order to keep constant the tension in a power transmission belt such as a timing belt or an accessory machinery driving belt for an automobile, prolong the durability of such a belt, and minimize noise during belt travel, a belt tension adjusting device as shown in FIGS. 1 and 2 is generally used.
For example, with a belt tension adjusting device for a cam-driving timing belt in an automobile, a bifurcated arm 4 (also referred to as a pulley arm) is pivotally supported on a fulcrum shaft 1 having a collar 1a and fixed to e.g. an engine block by a bolt 2 through a cylindrical slide bearing 3. A tension pulley 7 is mounted on a rotary shaft 5 provided at one end of the arm 4 so as to engage a belt 6 used in a transmission mechanism (not shown).
The arm 4 has its other end in abutment with a piston rod 9 of a damper 8 provided with a well-known damper mechanism in a hydraulic cylinder. The arm 4 is pivotally supported by a predetermined resilient biasing force of the piston rod 9 to remove slackening of the belt 6 by pressing the tension pulley 7 against the belt and to give it a suitable tension so that the belt transmission mechanism can work more efficiently.
With the cylindrical slide bearing 3 of the belt tension adjusting device, which is in slide contact with the outer peripheral surface of the fulcrum shaft 1, the surface pressure applied to its sliding surface (or inner peripheral surface) is large, because the tension in the belt 6 and the biasing force from the damper 8 are transmitted through the arm 4 and small-amplitude pivoting force acts on the arm from the belt 6, so that a load changes. Also the moment load acting on the arm due to a change in the tension in the belt 6 is loaded at the contact portion between the fulcrum shaft 1 and the slide bearing 3. These loads act not uniformly over the entire contact portion but as uneven loads. Also, the slide bearing 3 is a part that is liable to suffer seizure and wear because with the engine heat transmitted, the sliding surface becomes hot and various factors act in a composite manner.
With such a slide bearing 3, even if it is made of a material superior in wear resistance, protrusions on the surface at the sliding portion tend to peel in relatively large units, and the peeled masses sometimes act as abrasives. Thus, wear of the slide bearing is promoted.
Also, for the material of the slide bearing, physical properties and wear resistance that prevent the bearing material from adhering to the fulcrum shaft 1 of the arm 4 are required. Heat resistance is also required to maintain mechanical strength at high temperature.
In order to satisfy these requirements, in conventional belt tension adjusting devices, slide bearings made of a condensed type aromatic polyimide resin, which has a good heat resistance, have been used.
But with a belt tension adjusting device in which the fulcrum shaft of the pulley arm is supported by a slide bearing made of a condensed type aromatic polyimide resin, since molding of a polyimide resin was not easy, it was difficult to mass-produce slide bearings and thus to achieve improved production efficiency and low cost.
With a reduction in size of belt tension adjusting devices, the surface pressure applied to the slide bearing of the fulcrum shaft increased so much as to make it impossible to sufficiently prevent wear of the slide bearing.
Furthermore, if a belt tension adjusting device is mounted in the engine room of an automobile, this means that the slide bearing is used at around from 80xc2x0 C. to 150xc2x0 C. Thus it was impossible to maintain required mechanical strength in such a heated state because bearing material tends to adhere to the fulcrum shaft and it tends to be worn.
An object of this invention is to provide a belt tension adjusting device in which even if the slide bearing is heated to a high temperature by heat of an automotive engine, it is less likely to get worn and the fulcrum shaft will not seize.
Another object of this invention is to provide a belt tension adjusting device in which the pivoting resistance of the arm is sufficiently small, the amount of wear of the slide bearing is small, and which has a slide bearing made of an injection-moldable synthetic resin to cope with the requirement for a low cost.
According to this invention, there is provided a belt tension adjusting device comprising a stationary fulcrum shaft, an arm pivotally supported on the fulcrum shaft through a slide bearing formed of a synthetic resin, a tension pulley mounted on the arm at one end thereof for tensioning a belt, a damper having a piston rod abutting the arm on the other end thereof, the tension pulley being pressed against the belt by pivoting the arm under the force of the piston rod, whereby adjusting the tension of the belt, characterized in that the inner peripheral surface of the slide bearing, which is in slide contact with the fulcrum shaft, is formed so as to have a surface roughness RA of not more than 6.3 xcexcm.
In view of the working cost and attacking properties on the slide bearing, the surface roughness Ra should be 0.1-1.6 xcexcm. The material of the fulcrum shaft is not particularly limited if it can withstand the use as the fulcrum shaft. But Ni-plated steel, which is superior in fretting and seizure resistance, and stainless steel or a ceramic material, which has good corrosion resistance, are preferable.
According to this invention, the slide bearing may be formed of a resin composition comprising 65-85 vol % of a thermoplastic polyimide resin expressed by the formula 2, an aromatic polyether ketone resin or a mixture of these resins, and 15-35 vol % of a tetrafluoroethylene resin. 
(In the formula, X represents a direct bond or a group selected from the group consisting of hydrocarbon group having a carbon number of 1-10, hexafluorinated isopropylidene groups, carbonyl groups, thio groups and sulfonic groups, and R1-R4 represent hydrogen, lower alkyl groups having a carbon number of 1-5, lower alkoxy groups having a carbon number of 1-5, chlorine or bromine and may be identical to or different from one another. Y represents a quadrivalent group selected from the group consisting of an aliphatic group having a carbon number of 2 or more, a cycloaliphatic group, monocyclic aromatic group, condensed polycyclic aromatic group, and non-coupled polycyclic aromatic group obtained by coupling aromatic groups directly or through crosslinking members.)
Also, in the belt tension adjusting device, the resin composition may contain 2-15 vol % of an aromatic polyamide resin. Further, it may contain 1-5 vol % of graphite.
With the belt tension adjusting device of this invention, because the inner surface of the slide bearing for the fulcrum shaft has a roughness Ra of not more than 6.3 xcexcm, and the slide bearing is formed of a resin composition in which one or both of a thermoplastic polyimide resin and an aromatic polyether ketone resin, which are good in heat resistance and injection moldability, and a tetrafluoroethylene resin, which is superior in sliding properties, are mixed in predetermined amounts, or a resin composition in which a predetermined amount of an aromatic polyimide resin and/or a predetermined amount of graphite is further added, even if the heat of the engine is conducted, so that the sliding surface of the fulcrum shaft portion becomes hot, seizure will not occur. A belt tension adjusting device is thus provided which is sufficiently small in the resistance during pivoting of the pulley arm, small in wear and has a long life.
Also, since the belt tension adjusting device is equipped with a slide bearing made of an injection-moldable synthetic resin, manufacturing efficiency is high and thus it can cope with the requirement for a low cost.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: